The present invention relates to a method of controlling an output/input speed ratio of a continuously variable transmission (CVT) in which primary and secondary pressures are controlled.
Continuously variable automatic transmissions, also called CVT, for motor vehicles usually include a first cone pulley pair on an input shaft as a primary pulley set and a second cone pulley pair on an output shaft as a secondary pulley set. Each cone pulley pair consists of a first axially stationary pulley and a second axially movable pulley. Between the cone pulley pairs rotates a belt or torque transmission member which is wound around the cone pulley pair.
The running radius of the torque-transmitting belt may be adjusted by adjustment of the cone pulley pairs. Adjustment of the cone pulley pairs, accordingly, adjusts the running radius of the pulley at the input shaft and output shaft, thereby adjusting the output/input speed ratio of the CVT.
In order to adjust the primary or secondary pulley, the respective axially movable pulley is actuated with a pressure medium from a pressure source.
In a typical prior art CVT system, a stepper motor is used to actuate a ratio control valve to control the primary pressure associated with the cone pulley pair at the input shaft. By way of example, a look-up table may be used to control the stepper motor position versus the primary pressure. Accordingly, the primary pressure is not directly monitored or controlled, but rather the stepper motor is directly controlled leading to an indirect control mechanism. The physical delay in the response of the cone pulleys to the pressure input will not allow high control gains, thereby limiting the transient response.
The present invention provides an improved method of controlling the output/input speed ratio of a CVT by using the force ratio as the controlled variable. The control algorithm uses logic-based switching to boost either the primary or secondary pressure and incorporates dead time compensation along with a model of the CVT plant to overcome delay-induced instability. The controller is a proportional integral (PI) controller having integral anti-windup compensation.
More specifically, one aspect of the invention provides a method of controlling the output/input speed ratio of a continuously variable transmission (CVT) (also called the CVT plant herein) which includes primary and secondary pulleys actuated by primary and secondary hydraulic pressures (PP,PS), respectively. The pressures generate forces to move the pulleys and adjust a corresponding belt, thereby adjusting the output/input ratio. The method includes calculating a desired force ratio of the CVT plant. Desired primary and secondary pressures (PP,PS) are then determined based, in part, upon the calculated desired force ratio, and PS and PP signals are sent to the CVT plant to control the operation of the CVT plant. The actual speed ratio is calculated based on input and output speed measurements from the CVT plant. An error signal is generated based upon the actual speed ratio for use in calculating a next sample time desired force ratio which is then used to calculate desired PS and PP signals to be sent as inputs to the CVT plant, thereby using the desired force ratio as the controlled variable in controlling output/input speed ratio of the CVT plant.
In accordance with a further aspect of the invention, a method is provided for controlling the output /input speed ratio of a CVT which eliminates the possibility of belt slip and also improves transient response. The method includes logic-based determination of which one of the primary and secondary pressures should be boosted. The minimum pressure at which the other one of the primary and secondary pressures may be set to avoid belt slippage is then determined at each time instant, and that minimum pressure is set accordingly. That one of the primary and secondary pressures which is determined to be boosted is then boosted to a higher desired pressure, thereby achieving quick output/input ratio adjustment while avoiding belt slippage.
In accordance with yet another aspect of the invention, a further method is provided for controlling the output/input speed ratio of a CVT which overcomes both instability due to plant delay and poor response due to actuator saturation. The method includes the following steps: (A) calculating desired PP and PS signals, providing the desired PP and PS signals to a CVT plant with inherent delay, and calculating actual CVT speed ratio (output speed/input speed) from measurements of the input and output speeds from the CVT plant; (B) converting the calculated desired PP and PS signals to a calculated force ratio, and inputting the calculated force ratio to a model of the CVT plant; (C) determining a model speed ratio from the model of the CVT plant, and adjusting the model speed ratio for time delay; (D) subtracting the adjusted model speed ratio from the actual CVT speed ratio to provide an adjustment signal; (E) adding the adjustment signal to the determined model speed ratio prior to the delay adjustment of the model speed ratio to provide a delay-adjusted plant speed ratio; and (F) subtracting the delay-adjusted plant speed ratio from a speed ratio reference value to provide an error signal, which is then used in the calculation of desired PP and PS values for a next sample time using a proportional-integral anti-windup compensator.